Nanophotonics has started to facilitate the replacement of bulky optical components by ultrathin, planar elements. The recent realization of metasurfaces, metamaterials of reduced dimensionality, is of particular relevance as it is opening up new opportunities to realize virtually flat optics. Metasurfaces can improve performances of conventional optical elements and facilitate the creation of entirely new functionalities, which were not explored yet. In particular, gradient metasurfaces are 2D optical elements capable of manipulating light by imparting local, space-variant phase-changes on an incident electromagnetic wave.
Various metasurfaces based on metallic and dielectric nano-antenna have been developed. Early work on metasurface based on V-shape antenna can shape the wavefront of one linear polarization. Recently, dielectric gradient metasurface optical elements (DGMOEs) capable of achieving high efficiencies in transmission mode in the visible has been demonstrated. Ultrathin gratings, lenses, and axicons have been demonstrated by patterning a 100-nm-thin Si layer into a dense arrangement of Si nanobeam-antennas, which serve as the ultrathin waveplates. By controlling the local orientation of the fast-axes of Si nanobeam-waveplates, the desired phase profile with 8-level discretization can be accomplished. It has been demonstrated that metasurfaces can do the optical wavefront shaping as well as conventional lens. Compared to other metasurfaces, DGMOE has the advantages of modulate phase in full a range while maintaining equal amplitude. Also DGMOE works for a broadband wavelength regime. See, for example, U.S. Pat. No. 9,507,064 entitled “Dielectric Metasurface Optical Elements” which is incorporated herein by reference. There remain certain limitations of the DGMOE technology in its current state. For example, metasurface lenses at visible wavelengths have been developed for imaging, but their use has been limited to single wavelength applications chiefly due to the large degree of chromatic aberration in the lens design. Although various efforts have been made to avoid this problem, achromatic imaging based on metasurfaces at visible wavelengths is still not available. Thus, it would be an advance if these and other limitations of the current state of the art could be overcome.